Method for connecting anodes



United States Patent US. Cl. 156-293 5 Claims ABSTRACT OF THE DISCLOSURE This disclosure relates to a method for bonding the metal connection member which carries current from the electrical source to the prebaked carbon anode used in an electrolytic reduction cell wherein the molten cast iron normally used is replaced by a furane mortar binder.

This invention relates to a method of preparing a prebaked carbon anode for use in an electrolytic reduction cell. More particularly, this invention relates to a method for bonding the metal connection member which carries current from the electrical source to the prebaked carbon anode.

In the production of aluminum by the conventional electrolytic process, the electrolytic cell comprises in general a steel cell having disposed therein a carbon lining. The bottom of the carbon lining along with a layer of electrically produced molten aluminum which collects thereon during operation serves as the cathode. One or more consumable carbon electrodes is disposed from the top of the cell and is immersed at its lower extremity into a layer of molten electrolyte which is disposed in the cell. In operation, the electrolyte or bath, which is a mixture of alumina and cryolite, is charged to the cell, and an electric current is passed through the cell, from the anode to the cathode via the layer of molten electrolyte while oxygen collects at the anode. A crust of solidified electrolyte and alumina forms on the surface of the bath, and this is usually covered over with additional alumina.

In the conventional electrolytic process, use has been made of two types of electrolytic cells, namely, that commonly referred to as a prebaked" cell and that commonly referred to as a Soderberg cell. With either cell, the reduction process involves precisely the same chemical reactions. The principle difference is one of structure. In the prebaked cell, the carbon anodes are prebaked before being installed in the cell, while in the Soderberg cell or self-baking anode cell, the anode is baked in situ, that is it is baked during operation of the electrolytic cell, thereby utilizing part of the heat generated by the reduction process. The instant invention is particularly applicable to the prebaked cell.

The standard present day method of providing electrical contact between the prebaked carbon anode and the source of current uses a metal connection member. A cavity is formed in the top portion of the anode larger than the stub or lower end of the metal connection member. The metal connection member may be of one piece construction, i.e., the stub and rod portion being one continuous piece, or the stub may be a separate piece from the rod portion and connected to the rod portion in a suitable manner, for example by bolting or by welding. Similarly, the rod portion and the stub portion of the metal connection member may be of the same or ditferent metals. For example, the rod portion may be of copper or aluminum and the stub portion of steel, copper or aluminum. The stub of the metal connection member is placed in the cavity and molten cast iron poured around the stub. The cast iron is allowed to set and solidify. When the joint has achieved suflicient strength, the anode and connection member assembly may then be moved and as needed placed in service in the electrolytic reduction cell.

The present day practice, although generally quite satisfactory does have some disadvantages. The iron must be melted in order to be poured and cast in the cavity around the stub. It would be desirable to eliminate or substantially reduce the cost of melting the iron. Should an anode crack or break in use there is a possibility of iron contamination of the electrolytic melt. Moreover, when a spent anode is removed from an operating cell, the butt portion of the anode is frequently saved and ground up for use in forming new anodes or in forming cell linings. Here again, it is possible that some of the iron will adhere to the carbon particles of the anode and could eventually result in undesirable iron contamination of the electrolytic cell.

In the method according to the instant invention, the molten cast iron is replaced by a furane mortar binder. This has several advantages over the prior art practice. The mortar or binder is a pure carbon material and hence is non-contaminating if lost in the cell or recycled with anode butt material. It is also an advantage of the instant invention that the method for preparing the binder is a safer, simpler, lower temperature operation than melting lron.

The furane binder is essentially a mixture of furfuryl alcohol resin with a graphite aggregate. Furfuryl alcohol resins are well known and commercially available from many sources. The methods for preparing furfuryl alcohol resins are also well known in the art. A typical process for making a furfuryl alcohol resin might, for example, involve placing a given amount of furfuryl alcohol together with 10 percent by weight of an aqueous solution of phosphoric acid (0.5% of acid based on the alcohol) in a 3 necked round bottomed flask equipped with a stirrer, thermometer and reflux condenser. The stirrer is started and the mixture heated at reflux for several hours or until a desired viscosity is attained. If a liquid resin having good shelf life is desired, the acid catalyst may be neutralized (for example, with aqueous sodium hydroxide or other base), the water layer decanted and the product distilled in vacuum. Under these conditions, a relatively anhydrous fluid resin is obtained in yields, depending upon the viscosity, of 80 to percent by weight (based on the alcohol).

According to the instant invention, a graphite aggregate of suitable particle size distribution for the binder mix is prepared by impregnating the aggregate with an organic acid catalyst. Any suitable size range may be used for the graphite aggregate. In general for normal reduction cell anode usage, it is desirable that the aggre gate contain no plus 8 Tyler screen mesh particles since particles of that size would interfere with the mortar flowability. A representative graphite aggregate particle size distribution is shown in Table I.

3 TABLE I Graphite aggregate,

Tyler screen fraction: percent retained The even distribution of size fractions shown in Table I permits high mortar density by particle dovetailing. The amount of aggregate used is geared to the width of the gap between the stub and the cavity in the anode. For very tight fitting joints, 50 percent aggregate might be used. For the standard commercial electrolytic reduction cell, 70 to 75 percent aggregate is used. For a very wide joint, larger amounts, for example 80% of aggregate would be used. The aggregate besides being used as a catalyst carrier is also used to control mortar shrinkage during use of the anode in the electrolytic reduction cell. The more highly calcined stable aggregate that is used, the less the binder tends to shrink as it bakes during operational use of the anode.

The acid catalyst that is used must be an organic acid in order to prevent the introduction of non-carbon materials to the mixture. Suitable organic acids for this use are aryl sulfonic acids, aryl carboxylic acids, alkyl carboxylic acids, alkyl sulfonic acids, alkyl dicarboxylic acids, and aryl dicarboxylic acids or mixtures thereof. Typical examples of such acids which can be used are Ibenzene sulfonic acid, toluene sulfonic acid, naphthalene sulfonic acid, benzoic acid, salicylic acid, acetic acid, propionic acid, propane sulfonic acid, butane sulfonic acid, maleic acid, oxalic acid, malonic acid, phthalic acid, napthalic acid, analine hydrochloride, or mixtures thereof.

Twenty to fifty percent by weight of furfuryl alcohol resin is mixed with the graphite aggregate impregnated with the organic acid catalyst to form a paste. The relative proportions of furfuryl alcohol resin and aggregate will vary as indicated above depending upon the particular application. This paste is then applied to the cavity in the top portion of the anode. The metal stub of the metal connection member is then heated to-an elevated temperature, for example, from 100 C. to 250 C., by any suitable means. The heated metal stub is then inserted into the cavity to displace the paste in the cavity up and around the stub. The temperture of the stub is sufiicient to trigger the catalytic polymerization of the furfuryl alcohol resin. A strong adherent contour fitted electrically conductive bond results bonding the metal stub to the anode. In less than 5 minutes the joint is strong enough for the anode metal connection member assembly to be lifted and moved. When placed in operation in an electrolytic reduction cell, the binder will *bake out from the heat generated in the process and reach its ultimate strength.

sembly, and an intentionally incorrected spaced furane bonded anode assembly.

TABLE II.SIUB-CARBON RESISTANCE (MICROHMS Days of anode operation Standard anode 98 77 51 97 72 87 74 77 81 72 82 78 78 69 57 70 Correctly spaced furane- 83 86 96 97 97 114 bonded anode. 81 99 93 Incorrectly spaced I'urane- 116 160 bonded ano e.

For these tests the furane binder was made by blending one weight of furfuryl alcohol resin with 1 and 1% Weights of graphite aggregate. The graphite aggregate was impregnated with parabenzene sulfonic acid. The ingredients were cold mixed to form a paste. Approximately 1.3 pounds of this mixture which had a consistency of peanut butter was placed in the cavity in the carbon anode. The metal connection member which had a steel stub bolted to a copper bar or rod was then heated until the steel stub was at a temperature of 200 C. The stub was then hand pressed into the cavity displacing the fluid binder mixture up and around the stub filling the stub carbon joint. The heat from the stub triggered the catalytic polymerization of the furfuryl alcohol resin binder causing it to become firm enough to support the anode weight in a few minutes. The anode was then placcdin a cell in the normal manner where the furane binder joint became thoroughly cured and electrically conductive as the anode heated up and carried its electrical load.

As can be seen in Table II, the anode assembly having the furane binder correctly mixed for the spacing between the stub and the sides of the cavity in operation had substantially the same stub to carbon resistance as the standard cast iron bonded anode assembly. One anode assembly was purposely prepared wherein the mixture was not proper for the gapping between the stub and the cavity sidewalls. That is, this anode stub was purposely incorrectly spaced in the cavity. As can be seen in Table II, the tight furane joint failed after 5 days. In these tests, the above indicated mixture was shown to be proper for a 5 plus or minus spacing around a 4" diameter steel stub. The shrinkage of the furane mortar equalled the thermal expansion of the steel closely enough than neither loosening nor bursting of the stubanode joint occurred in operation.

In another test run utilizing the same aggregate and furane alcohol resin mixture as a binder and a slight excess of catalyst, the heated stub was inadvertently positioned 90 out of the desired position. After only 30 seconds of set time the stub could not be budged by two men with a steel bar. This indicates that a tight strong bond is very rapidly formed according to the practice of this invention.

Table III illustrates the properties of two difi'erent blends of furane mortar binder.

TABLE III Furauc mortar mixes (parts by weight) Properties with temperature curing Furturyl Carbon Coarse Percent Av. Shrinkage alcohol filler+ carbon Wt. loss, Elec. Res, lineal coetl. resin catalyst filler Temp, C. percent SZ-cm. shrinkage 10 C.

Table II shows a comparison between a furane bonded In Table III the carbon filler plus catalyst is that anode assembly, a standard cast iron bonded anode as- 75 shown in Table I. The coarse carbon filler used in one test and shown in Table III was a minus 4 and plus 8 mesh Tyler screen fraction of graphite. It can be seen from this table that the mixture having more coarse carbon graphite aggregate had a lower shrinkage coeflicient than the other mixture tested for purposes of comparison. This indicates that the binder composition and shrinkage can be tailored to the specific application, i.e., to the width of the gap between the stub and the sides of the cavity and the steel expansion.

1 Cast iron.

Table IV shows the results of cell testing of various furane stub joints in comparison with a standard cast iron joint. In these tests, the aggregate used was that shown in Table I with only the minus 200 Tyler screen fraction of the aggregate being impregnated with the organic acid catalyst. The particular organic acid catalyst used was paratoluene sulfonic acid. The results of the tests as shown in Table IV illustrate that the stub to carbon resistance is inversely proportional to the metal to carbon contact area for both cast iron and furane binder joints. Thus, from the standpoint of electrical resistivity, it appears immaterial what the metal is (for example a steel stub or the cast iron hinder), or whether the carbon is baked anode material or the furane graphite binder, provided the contact areas are similar and well constructed.

Thus, it can be seen that the instant invention provides a method of binding the metal connection member to the prebaked carbon anode for use in an electrolytic reduction cell with no increase in the electrical resistivity of the joint, with many attendant advantages. The cost of preparing and making the joint between the metal connection member and the prebaked carbon anode with proper engineering is less than for cast iron where the iron must be melted and heated to the desirable temperature. The binder is a pure carbon material and hence is noncontaminating if lost in the cell or recycled accidentally with anode butt material. The instant method is a safer, simpler, lower temperature operation than that using molten cast iron,

It is apparent that various changes and modifications may be made without departing from the invention, wherein what is claimed is:

1. In the method of preparing a prebaked carbon anode for use in an electrolytic reduction cell by bonding the metal stub of the metal connection member of an anode rod to the carbon anode in a prepared cavity in the top portion of the anode, the improvement comprising:

(a) impregnating a graphite aggregate with an organic acid catalyst;

'(b) mixing 20% to 50% by weight of a furfuryl alcohol resin with the graphite aggregate to for-m a paste;

(c) applying the paste to the cavity in the top portion of the anode;

(d) heating the metal stub of the metal connection member to an elevated temperature;

(e) inserting the heated metal stub into the cavity to displace the paste in the cavity up and around the stub, the temperature of the stub being sufiicient to trigger the catalytic polymerization of the furfuryl alcohol resin, whereby a strong, adherent contour fitted electrically conductive bond is formed bonding the metal stub to the anode.

2. The method of claim 1 wherein the graphite aggregate particles are all 8 Tyler screen size and only the 200 Tyler screen fraction is impregnated with catalyst.

3. The method of claim 1 wherein 25% to 30% by weight of furfuryl alcohol resin is mixed with the aggregate.

4. The method of claim 1 wherein the organic acid is selected from the group consisting of arylsulfonic acids, arylcarboxylic acids, and alkyldicarboxylic acids.

5. The method of claim 1 wherein the metal stub is heated to from C. to 250 C.

References Cited UNITED STATES PATENTS 2,698,319 12/1954 Brown et a] 260--88.5 3,184,814 5/1965 Brown 164-12 2,601,497 6/1952 Brown 26067 2,817,620 12/1957 Golick et al 156-294 2,937,980 5/1960 Schmitt et al. 204-67 3,198,714 8/1965 Johnson et a1 l0656 X 3,201,330 8/1965 Price et a1 106-56 X 3,303,119 2/1967 Dell 204-290 3,334,040 8/1967 Conrad et al 204-266 HAROLD ANSHER, Primary Examiner U.S. Cl. X.R. 

